This website uses cookies to deliver some of our products and services as well as for analytics and to provide you a more personalized experience. Click here to learn more. By continuing to use this site, you agree to our use of cookies. We've also updated our Privacy Notice. Click here to see what's new.

This website uses cookies to deliver some of our products and services as well as for analytics and to provide you a more personalized experience. Click here to learn more. By continuing to use this site, you agree to our use of cookies. We've also updated our Privacy Notice. Click here to see what's new.

About Optics & Photonics TopicsOSA Publishing developed the Optics and Photonics Topics to help organize its diverse content more accurately by topic area. This topic browser contains over 2400 terms and is organized in a three-level hierarchy. Read more.

Topics can be refined further in the search results. The Topic facet will reveal the high-level topics associated with the articles returned in the search results.

Abstract

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.6. For such small Fresnel numbers, it may be possible to eliminate the lenslets entirely, replacing them with a simple mask containing a sparse array of clear subapertures and thereby also eliminating a number of manufacturing problems and experimental complications associated with lenslets. We present laboratory results that demonstrate the validity of this approach.

References

You do not have subscription access to this journal. Citation lists with outbound citation links are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution.

Table 1

Summary of the Seven Narrowband Laboratory Fresnel Phasing Runs from the Current Investigation with Two Typical Fraunhofer Phasing Runs from the Keck Telescope Shown for Comparisona

Date

Mask

λ/Δλ (nm)

Turb.

Pix per Arcsec

Pix per λ/d

F

Excluded Edges (%)

Error (nm)

10 Feb 1995

Keck

651/35

yes

5.33

5.96

0

0

5.6

10 Feb 1995

Keck

891/10

yes

5.33

8.16

0

0

4.1

18 Mar 2011

Kapton

660/0

no

2.35

2.67

0.73

0

5.8

21 Mar 2011

Kapton

650/10

yes

2.35

2.63

0.60

3.8

8.5

21 Mar 2011

Kapton

900/100

no

1.97

3.05

0.68

4.7

11.5

25 Mar 2011

chrome

660/0

no

2.25

2.55

0.69

0

4.7

25 Mar 2011

chrome

650/10

no

2.25

2.51

0.69

0

4.8

25 Mar 2011

Kapton

650/10

yes

2.35

2.63

0.63

0.7

10.1

25 Mar 2011

chrome

900/100

no

1.86

2.88

0.72

0

5.6

a The error column refers to the RMS edge step (surface) averaged over intersegment edges for the 11-step measurement process. The Fresnel errors are 1.5 times larger than Fraunhofer on average. This is believed to result not from any fundamental difference between Fresnel and Fraunhofer phasing, but from the less favorable image scale (especially in pixels per λ/d) and from the poorer mask-pupil registration available in the laboratory Fresnel setup. The column labeled F gives the Fresnel number of the apertures (F=0 corresponds to Fraunhofer diffraction). Some edges were excluded on the basis of a χ2 fit, as discussed in the text. The same exclusion algorithm is in place at Keck, although for this Keck data, as is often the case, no edges were in fact excluded.